In order to prevent transient ischemic episodes, coronary blood flow (O2 delivery) must closely match O2 use of the myocardium not only in steady state situations, but also in dynamic situations. We propose to study the mechanisms responsible for the local control of coronary blood flow with special emphasis on the control of dynamics of coronary vascular resistance in response to sinusoidal variation in heart rate. Coronary vascular resistance, myocardial O2 consumption and coronary sinus pO2 responses to sinusoidal variations in heart rate will be observed at different mean coronary blood flow, arterial pO2's and after K ion depletion, and after administration of aminophylline, liodflazine, indomethacin and meclofenamate. Changes in the dynamics of coronary vascular resistance will provide a sensitive indication of the participation of pO2, adenosine, K ion, and prostaglandins in the control of coronary vascular resistance in phasic and/or steady state conditions. A possible myogenic response to extravascular ventricular compression will be evaluated in another series of experiments. Emphasis on the dynamics of coronary blood flow control is necessary in order to uncover both phasic and redundant steady state control mechanisms; in addition such experiments may provide information concerning the limits of O2 supply to the heart during sudden initiation of exercise or emotional stress.